The eruption began at 9:20 AM, as witnessed by the
onset of a continuous eruption tremor recorded by seismographs, and ten
minutes later an eruption plume was observed. Photographs show the
plume from various distances, up to 200 km (the first image), from 11:20
AM until 12:50 PM. The plume was continuous and extended to elevation
of 10 km (32000 feet), loaded with tephra. The plume was deflected
to the SE, by mild westerly winds. The eruption occurred at the southern
edge of the Grímsvötn caldera, at the foot of Mt. Grímsfjall
(which rises about 300 m above the flat ice shelf of the Grímsvötn
subglacial lake). The eruption penetrated through the Grímsvötn
subglacial caldera lake and its ice shelf, from ice/water depth of about
100 meters. The eruptive fissure was about 1300 m long, east-west
directed, along the Grímsvötn caldera fault. Activity
was most vigorous at one crater, but several other craters on the short
eruptive fissure were also active with less frequent explosions.
Last image gives a good overview over the eruptive site. It is taken
from elevation of 10 km, near the top of the eruption plume, and gives
a view towards the NE. It shows the location of the eruptive site
at the southern boundary of the Grímsvötn caldera, the tephra
sector towards the SE, the northern boundary of the Grímsvötn
caldera, and ice cauldrons over the 1996 Gjálp eruptive site, north
of Grímsvötn. The width of the Grímsvötn
caldera is about 10 km.

19 December 1998 - Day 2 of the Eruption

The eruption was slightly less vigorous than on 18th
of December. The eruption plume was continuous, but somewhat lower
than the day before, extending to an elevation of 7-8 km height (23000
-27000 feet). Photographs show the eruption between 10:30 AM and
12:30 PM. Tephra continued to fall towards the SE. A small
part of the Grímsvötn ice shelf, next to the eruption site,
had melted, without raising the water level of the Grímsvötn
caldera lake significantly. Activity was mostly limited to one crater.

20 December 1998 - Day 3 of the Eruption

Flight over the eruptive site between 10:45 AM and
12:15 PM revealed variable activity. Eruption plume extended highest
to 7 km elevation (22000 feet). Initially the plume was rater light
colored, and narrow at its base. The eruptive site and Mt. Grímsfjall
could be clearly seen (first image). Later the ash content of the
plume greatly increased, and the plume turned black. It collapsed
(become lower) by several thousands feet (1-2 km), created a base surge,
and Mt. Grímsfjall disappeared into an ash cloud.

27 December 1998 - Day 10 of the Eruption

This was the second last day of the eruption, it ended
on December 28. Photographs show intermittent eruptive activity between
11:24 AM and 12:40 PM. The eruption plume was discontinuous, fed
by intermittent activity in the crater. It rose highest to about
4.5 km elevation (15000 feet), but disappeared in-between. The plume
distributed ash in the near vicinity of the crater, and ejected bombs,
up to 0.5 m in diameter onto Grímsfjall. Intervals of low
activity allowed good visibility towards the active crater, and the water
pond NE of the eruptive site, formed by melting of the Grímsvötn
ice shelf. Although not clear from the photographs shown here, the
eruption has resulted in the formation of a tephra ring that lies partly
on ice, but its inner part is likely to be made completely of ash overlying
bedrock in the Grímsvötn depression. It is likely to
consolidate into a tuff ring, which may eventually exist as a nunatak in
the ice cap.